The present invention generally relates to semiconductor devices, and more particularly to a semiconductor device applicable to a semiconductor laser having a GaAs/AlGaAs system compound semiconductor heterojunction and capable of easily obtaining an arbitrary light emission wavelength over a wide range.
Semiconductor lasers and light emitting diodes generate to the outside a light which is generated at a time of a recombination, that is, a light emitting phenomenon which occurs due to an electron transition in an energy gap between the valence and conduction bands. However, the light emission wavelength of such semiconductor lasers and light emitting diodes is determined by an energy gap peculiar to the material used. For this reason, it is necessary to produce a mixed crystal semiconductor including three or four elements in order to obtain a desired light emission wavelength especially over a wide range, but it is difficult to obtain a satisfactory single crystal.
Accordingly, there is a demand to realize a semiconductor device capable of easily obtaining an arbitrary light emission wavelength over a wide range and obtaining a satisfactory single crystal.
On the other hand, it is possible to produce a superlattice, that is, a periodic repetition of different semiconductors like AlGaAs/GaAs/AlGaAs/GaAs by use of a crystal growing method such as a molecular beam epitaxy (MBE) which can control a thickness of a grown layer with a high accuracy. There has been reports that a band structure of the conduction band in such a superlattice splits into mini-bandgaps.
FIG. 1 shows a cross sectional structure of an example of a conventional semiconductor laser, and FIG. 2 is a diagram for explaining trapping of carriers in the conventional semiconductor laser shown in FIG. 1. FIG. 3 is a diagram for explaining a band structure and a distribution function of the conventional semiconductor laser shown in FIG. 1, and FIGS. 4A and 4B are diagrams for explaining band structures of a bulk GaAs and a superlattice.
In FIG. 1, the semiconductor laser has a p-type AlGaAs cladding layer 1, a p-type GaAs active layer 2, an n-type AlGaAs cladding layer 3, and electrodes 4. In FIGS. 2 and 3, Eg denotes an energy band gap, E denotes an energy, k denotes a wave number, E.sub.FC denotes a Fermi level in a conduction band, E.sub.FV denotes a Fermi level in a valence band, Ea denotes an energy at a top of the valence band, Eb denotes an energy at a bottom of the conduction band, f(E) denotes a distribution function of electrons, fc(E) denotes a distribution function of electrons in the conduction band, fv(E) denotes a distribution function of electrons in the valence band, fc(Eb) denotes a distribution function of electrons at the energy at the bottom of the conduction band, fv(Ea) denotes a distribution function of electrons at the energy at the top of the valence band, .sigma.(E) denotes a density of states, .sigma..sub.c (E) denotes a density of state in the conduction band, and .sigma..sub.c (E) denotes a density of state in the valence band. In FIGS. 4A and 4B, 5a and 5 b respectively denote mini-bandgaps formed in the conduction band and corresponding to forbidden bands, 6 denotes a band structure of a bulk GaAs, and 7 denotes a band structure of a superlattice made up of GaAs/AlGaAs/ . . . /GaAs.
A description will now be given of the operating principle of the conventional semiconductor laser shown in FIG. 1. As shown in FIG. 2, the electrons and holes are trapped in the same space in the energy bandgap Eg between the valence and conduction bands and the semiconductor laser uses a light hv generated at a time of a recombination of the electrons and holes. The electrons and holes are successively injected by applying a voltage across the electrodes 4. In addition, as shown in FIG. 3, the condition for amplification for continuous light emission is that a population inversion occurs. The condition may be expressed by the following, where B is a transition rate for stimulated emission and .rho. is the photon number. EQU B.rho..intg..sigma..sub.c (E).sigma..sub.v (E-hv){fc(E)-fv(E-hv)}dE&gt;0
Since .sigma..sub.c (E) and .sigma..sub.v (E) are always positive numbers, a relation fc(E)&gt;fv(E-hv) is obtained. This condition of inequality is satisfied when Eg&lt;hv&lt;EFC-E.sub.FV.
For example, Esaki et al., "Superlattice and Negative Differential Conductivity in Semiconductors", IBM Journal of Research and Developments, pp. 61-65, January 1970 discloses an example of the superlattice. But no reports are made on semiconductor lasers and light emitting diodes.
Next, a description will be given of the superlattice. It is possible to produce a superlattice, that is, a periodic repetition of different semiconductors like AlGaAs/GaAs/AlGaAs/GaAs, by use of a crystal growing method such as the MBE which can control a thickness of a grown layer with a high accuracy. In this case, the band structure of the conduction band does not become a continuous energy distribution like the band structure 6 of the bulk GaAs shown in FIG. 4A but becomes a discontinuous energy distribution like the superlattice band structure 7 shown in FIG. 4B including the mini-bandgaps 5a and 5b.
However, according to the semiconductor laser shown in FIG. 1, the light emission wavelength is determined by the energy bandgap Eg peculiar to the material used. For this reason, it is necessary to use a mixed crystal semiconductor including three or four elements in order to obtain a desired light emission wavelength especially over a wide range, but there is a problem in that it is difficult to obtain a satisfactory single crystal.